755
FXUS63 KARX 151143
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
643 AM CDT Sun Sep 15 2024

.KEY MESSAGES...

- Warm temperatures persist through much of this week with high
  temperatures in the 80s for much of the region.

- Relatively dry conditions are expected through the first half
  of the week. Precipitation chances return starting Wednesday
  with increasing chances as you head further west.

&&

.DISCUSSION...
Issued at 325 AM CDT Sun Sep 15 2024


Today - Tuesday: Patchy Fog This Morning, Otherwise Above Normal
Temperatures With Minimal Precipitation Chances

Patchy fog will be possible this morning in portions of the area
with dewpoint depressions lowering throughout the overnight. While
surface winds are generally on the lighter side in recent RAP/NAM
soundings, winds just off the deck are slightly stronger to around
10-15 kts by 09z which should work as a mitigating factor for fog.
That being said, have noted some under 1 mile visibilities at Black
River Falls, Winona, and La Crosse early this morning so certainly
could be locally dense early this morning, particularly in areas
that more routinely see fog (cranberry bogs/river valley locations).
As we head later into the day, some moisture remains in place with
precipitable waters of around 1.25" per the 15.03z RAP and
instability reaching around 1000 J/kg of MLCAPE. However, with no
clear forcing mechanism to provide any lift to get showers/storms
going, any convective development would be very isolated in nature
(less than a 15% chance at any given location) and likely would
result in minimal QPF with the recent HREF having very low
probabilities (0-30%) for measurable precipitation today.

Otherwise, upper-level ridging will be the name of the game over the
next few days with the general pattern featuring an upper-level
high sitting east of the region and a developing trough moving
into portions of the Rockies. In this setup, current
deterministic guidance (15.06z GFS/EC/NAM) keeps our region
subjected to upper-level ridging with southwesterly flow
developing to our west. Consequently, expecting generally warm
conditions to persist as the grand ensemble (GEFS/EC
ensemble/Canadian ensemble) keeps median high temperatures
generally in the 80s with minimal inter-quartile spread (around
1 to 3 degrees of spread) into mid-week.

Wednesday - Sunday: Precipitation Chances Return While Remaining
Seasonably Warm

Looking later into the week, the aforementioned upper-level ridge
initially holds across our region with a large trough situated over
the Rockies keeping our temperatures fairly steady-state. However,
some pieces of shortwave energy try to sneak their way west of the
region along the eastern periphery of this upper-level trough. As a
result, noting some increasing precipitation chances in ensemble
guidance for western portions of the local area (southeast
MN/northeast IA) late week. While both the 15.00z GEFS and EC
ensemble have differing probabilities into Thursday/Friday, both
groups increase probabilities for measurable precipitation. The EC
ensemble is generally the more aggressive solution with higher
probabilities (50-80% as opposed to 20-40% in the GEFS) for
measurable precipitation on Thursday. A bit early to evaluate
exactly how thermodynamics will look late week but an initial look
at the deterministic 15.00z GFS would suggest minimal instability
building into our region with the stronger axis of 850mb moisture
transport remaining further west in addition to minimal shear as we
are still fairly removed from the stronger upper-level flow.
Therefore, when also considering that the grand ensemble joint
probabilities for 500 J/kg of SBCAPE and 30 kts of bulk shear being
very low (0-15%), the potential for severe weather currently appears
on the lower side late this week.

As we head towards the weekend, early indications show some
potential for the ridging pattern to finally break down as a larger
trough gets ejected towards the local area in the 15.06z GFS/EC. A
large degree of disagreement exist amongst various deterministic
models and ensemble groups on timing, position and strength of the
trough as it swings through. This can be highlighted with the 500mb
geopotential height inter-quartile range in the grand ensemble which
show roughly 10 decameters of spread for Sunday across the region, a
fairly substantial range of outcomes. Consequently, the previously
mentioned low inter-quartile spread in the grand ensemble for
surface temperatures observed through Friday gets tossed out the
window by the weekend. This can be noted at La Crosse where the
median high temperature for Sunday is 72 but the inter-quartile
range is anywhere from 67 to 79 degrees.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 639 AM CDT Sun Sep 15 2024

River valley stratus/fog remains the primary aviation concern
over the next 24 hours. At issuance time, stratus/fog is
present both north and south of the five mile radius of LSE.
With sunrise proceeding and winds starting to pick up, do not
expect this to fill in early this morning. Looking ahead to late
in the period, mid-level clouds overnight Sunday night and a
very shallow low wind surface layer may prevent fog formation,
so have left it out with this update. In the interim, some
cumulus at or around 5kft may develop today.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.

&&

$$

DISCUSSION...Naylor
AVIATION...Ferguson